1. Field of the Invention
The present invention relates to a light-emitting diode (LED) driving circuit. More particularly, the present invention relates to an LED driving circuit for driving a plurality of lightbars each including a plurality of LEDs coupled in series.
2. Description of the Prior Art
FIG. 1 is a block diagram illustrating a conventional LED driving circuit. Referring to FIG. 1, an LED driving circuit 1 is adapted to drive a plurality of lightbars LB1-LBm, with each lightbar LBi including a plurality of LEDs D1-Dn coupled in series, where m and n are positive integers, and i is an integer from 1 to m. The LED driving circuit 1 includes a direct-current to direct-current (DC/DC) converter 11 and an LED controller 12. The DC/DC converter 11, such as a buck or boost converter converts a DC input voltage Vin to a DC voltage V1b sufficient to drive the lightbars LB1-LBm. Each lightbar LBi has a first terminal coupled to the DC/DC converter 11 to receive the DC voltage V1b and a second terminal coupled to a corresponding channel terminal CHi of the LED controller 12. The LED controller 12 detects current of each lightbar LBi and controls current of each lightbar LBi to become equal to a predetermined value by built-in constant current sources or variable resistors. That is, the LED controller 12 balances currents of the lightbars LB1-LBm. The LED controller 12 further outputs a feedback signal from a feedback terminal FB to control the DC/DC converter 11 to adjust the DC voltage V1b. 
FIG. 2 is a block diagram illustrating another conventional LED driving circuit. Referring to FIG. 2, if too many lightbars LB1-LBm or LEDs D1-Dn with high brightness are employed, the total current of the lightbars LB1-LBm may cause the LED controller to be destroyed. Accordingly, there is a need for an LED driving circuit to employ an external control manner shown in FIG. 2. An LED driving circuit 2 includes a DC/DC converter 21, an LED controller 22, a plurality of transistors M1-Mm and a plurality of resistors R1-Rm. The transistor Mi and the resistor Ri are coupled in series between the second terminal of a corresponding lightbar LBi and ground. The LED controller 22 detects current of each lightbar LBi from a corresponding current sensing terminal ISi and outputs a signal from a corresponding channel terminal CHi to control current of each lightbar LBi to become equal to a predetermined value. The LED controller 22 further detects a voltage at the second terminal of each lightbar LBi from a voltage detecting terminal VDi to provide a short protection for the lightbars LB1-LBm.
The LED driving circuits 1 and 2 employ the LED controllers 12 and 22 which are specific-purpose integrated circuits (ICs). However, a commercially available LED controller IC supports a fixed number of lightbars. It may be necessary to employ a plurality of LED controller ICs to drive the lightbars as the number of the lightbars increases. The number of the transistors M1-Mm and the resistors R1-Rm employed in the LED driving circuit 2 will increase as the number of the lightbars increases. Therefore, as the number of the lightbars increases, the conventional LED driving circuits become more complex and expensive to design and manufacture.